Intersystem crossing via charge recombination in a perylene–naphthalimide compact electron donor/acceptor dyad

In order to study the relationship between the molecular structures of compact electron donor/acceptor dyads and the spin–orbit charge transfer intersystem crossing (SOCT-ISC) efficiency, we prepared a perylene (Pery)–naphthalimide (NI) dyad, in which the Pery unit is the electron donor and the NI unit is the electron acceptor, where the two units adopt an almost orthogonal geometry. The photophysical properties of the dyad were studied with steady state and femtosecond/nanosecond transient absorption (fs TA and ns TA) spectroscopies, time resolved electron paramagnetic resonance (TREPR) spectroscopy and DFT computations. A very weak charge transfer (CT) absorption band was observed, but the fluorescence of the Pery unit is strongly quenched. Upon selective excitation into the NI unit, the fast intramolecular CS process (10 ps) occurs, followed by ISC and only the 3Pery* state is observed; whereas upon selective photoexcitation into the perylene unit, an ultrafast charge separation (0.66 ps) is observed, followed by SOCT-ISC (8 ns) to populate the 3Pery* state. Moreover, the perylene radical cation is also observed on the ns timescale, presumably formed by intermolecular charge transfer. The lifetime of the 3Pery triplet state was determined to be τT = 214 μs with ns TA spectroscopy. The singlet oxygen quantum yield was measured as ΦΔ = 80%, although the potential energy curve of the torsion between the donor and acceptor is shallow. The SOCT-ISC mechanism was confirmed by TREPR spectroscopy. The dyad was used as a triplet photosensitizer for the generation of delayed fluorescence (luminescence lifetime τDF = 57.3 μs).